Hydrogen production from steam methane reforming coupled with in situ CO2 capture: Conceptual parametric study

The work deals with fundamental analysis of the sorption-enhanced steam methane reforming (SE-SMR) process in which the simultaneous removal of carbon dioxide by hydrotalcite-based chemisorbent is coupled. A two-section reactor model is developed to describe the SE-SMR reactor, decoupling the complexity in process analysis. The model defines two subsequent sections in the reactor: an equilibrium conversion section (upstream) and an adsorption reforming section (downstream). The material balance relationship in the equilibrium conversion section is directly determined by thermodynamic equilibrium calculation, providing an equilibrated atmosphere to the next section. The adsorption reforming section is described using an isothermal multi-component dynamic model into which the SMR reactions and the high-temperature CO₂ adsorption are embedded. The multiple requirements (including H₂ purity, H₂ productivity, CH₄ conversion enhancement, and carbon oxides concentrations) are taken into account simultaneously so as to analyze and define feasible operation window for producing high-purity hydrogen with ppm-level CO impurity. The performances of the reactors with different dimensions (laboratory-scale and pilot-scale) are explored, highlighting the importance of operation parameter control to the process feasibility.     

基于化学链燃烧的吸收剂引导的焦炉煤气水蒸气重整制氢过程模拟

建立了基于化学链燃烧供能的吸收剂引导的焦炉煤气水蒸气重整制氢系统,该系统包含吸收剂引导的焦炉煤气重整反应器（SECOGSR）、燃料反应器和空气反应器。该系统能产生高纯H₂［93.23%（mol）］,仅通过冷凝即可实现CO₂的捕获,分离能耗低。采用Aspen Plus软件对吸收剂引导的焦炉煤气重整制氢过程进行了模拟,得到优化的反应条件为：温度650℃,压力1.5 MPa,Ca/C=1,H₂O/C=4。并对系统进行了模拟,以NiO/Y₂O₃/ZrO₂(0.73/0.022/0.248,摩尔比）为化学链燃烧的载氧体和载能体,在满足反应器自热平衡和系统吸放热平衡的基础上,重整1mol焦炉煤气,燃料反应器和空气反应器所需的焦炉煤气、空气及载氧体NiO/Y₂O₃/ZrO₂的量分别为0.139、0.648、3.11 mol。该系统消耗1 mol焦炉煤气的产H₂量为1.30 mol,捕获的CO₂的量为0.355 mol。     

High-temperature attrition of sorbents and a catalyst for sorption-enhanced steam methane reforming in a fluidized bed environment

Mechanical degradation of the solid particles used in sorption-enhanced steam methane reforming (SE-SMR) was investigated in a gas jet attrition apparatus. The performance of a dolomite, a limestone and a commercial reforming catalyst were compared based on the air jet attrition index (AJI). The dolomite showed the poorest resistance to attrition, likely due to the extra pore volume caused by calcination of MgCO₃. The degree of loss of fines from the catalyst was significant, pointing to the need to develop catalysts suited to fluidized bed operation. Co-fluidization of the harder catalyst and the dolomite did not lead to additional attrition of the dolomite.     

Synthesis gas production using steam hydrogasification and steam reforming

Experimental work has been carried out on the mixed reforming reaction, i.e., simultaneous steam and CO₂ reforming of methane under a wide range of feed compositions and four different reaction temperatures from 700 °C to 850 °C using a commercial steam reforming catalyst. The experiments were conducted for a CO₂/CH₄ ratio from 0 to 2 and a steam to methane ratio from 3 to 5. The effect of CO₂/CH₄ ratio on the exit H₂/CO ratio and the conversions of the reactants indicate that the dry reforming reaction is dominant under increased carbon dioxide in the feed. Steam reforming of typical steam hydrogasification product gas consisting of CO, H₂ and CO₂ in addition to steam and methane has also been investigated. The H₂/CO ratio of the product synthesis gas varies from 4.3 to 3.7 and from 4.8 to 4.1 depending on the feed composition and reaction temperature. The CO/CO₂ ratios of the synthesis gas varied from 1.9 to 2.9 and 2.0 to 3.3. The results are compared with simulation results obtained through the Aspen Plus process simulation tool. The results demonstrate that a coupled steam hydrogasification and reforming process can generate a synthesis gas with a flexible H₂/CO ratio from carbon-containing feedstocks.     

Development of a Combined Plasma-Matrix Reformer for Solid Oxide Fuel Cell Application

A novel plasma-matrix reformer (PMR) was suggested for methane conversion into hydrogen-rich fuel. To demonstrate the possibility of reforming performance, characteristics of product gas and CH₄ conversion were identified according to O₂/C ratio, water vapor supply, reformed gas recirculation, and water feed in the recirculation gas affecting energy conversion and hydrogen production. When the reformed gas recirculation and water feed to the recirculation pipe were performed at the same time, hydrogen production and energy conversion efficiency were superior compared to the conventional reforming method. The optimal operating conditions of the PMR were determined. The obtained high energy conversion efficiency and hydrogen selectivity indicated the applicability to solid oxide fuel cell stacks for residential power generation.     

Adsorption of Off‐Gases from Steam Methane Reforming (H2, CO2, CH4, CO and N2) on Activated Carbon

Abstract

Hydrogen is the energy carrier of the future and could be employed in stationary sources for energy production. Commercial sources of hydrogen are actually operating employing the steam reforming of hydrocarbons, normally methane. Separation of hydrogen from other gases is performed by Pressure Swing Adsorption (PSA) units where recovery of high‐purity hydrogen does not exceed 80%.

In this work we report adsorption equilibrium and kinetics of five pure gases present in off‐gases from steam reforming of methane for hydrogen production (H₂, CO₂, CH₄, CO and N₂). Adsorption equilibrium data were collected in activated carbon at 303, 323, and 343 K between 0‐22 bar and was fitted to a Virial isotherm model. Carbon dioxide is the most adsorbed gas followed by methane, carbon monoxide, nitrogen, and hydrogen. This adsorbent is suitable for selective removal of CO₂ and CH₄. Diffusion of all the gases studied was controlled by micropore resistances. Binary (H₂‐CO₂) and ternary (H₂‐CO₂‐CH₄) breakthrough curves are also reported to describe the behavior of the mixtures in a fixed‐bed column. With the data reported it is possible to completely design a PSA unit for hydrogen purification from steam reforming natural gas in a wide range of pressures.     

A numerical study of multicomponent mass diffusion and convection in porous pellets for the sorption-enhanced steam methane reforming and desorption processes

Mass-based mathematical models have been formulated to describe the evolution of species mass fraction, pressure, velocity, density and mass diffusion flux in porous pellets for steam methane reforming (SMR) and sorption-enhanced steam methane reforming (SE-SMR) with CO₂ capture and desorption. The internal- and overall effectiveness factors have been calculated for the steam methane reforming, the sorption-enhanced steam methane reforming with a CaO-based adsorbent and the desorption processes. The accuracy of choosing the Wilke model to describe multicomponent diffusion instead of using the more costly Maxwell–Stefan- and Dusty gas models have been investigated. The different effects of choosing the random pore-, multi-grain- and the parallel-pore models have been investigated. Using an average size of the micro-particle the results obtained by in the multi-grain model, are slightly different than those for the parallel-pore model.The model evaluations revealed that:

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  The rate determining steps for the SMR process are basically the chemical kinetics, but the internal diffusion flux rate is also important as the main conversion takes place close to the external surface of the pellet. The SE-SMR process is basically chemical kinetics controlled. The approximate values of the efficiency factors for SE-SMR processes are around 1 and for the desorption it is around 0.02.

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  In general, the optimal diffusion flux model is the dusty gas model.

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  The multi-grain model is optimal pore model for the SMR and SE-SMR pellets.

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  There is a uniform temperature within the pellet.

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  The diffusion fluxes dominate over the convective fluxes. The pressure gradients and velocity vanish due to the imposed symmetry conditions.

     

Hydrogen production by sorption-enhanced steam methane reforming process using CaO-Zr/Ni bifunctional sorbent–catalyst

A bifunctional CaO-Zr/Ni (13, 18, and 20.5 wt% NiO) sorbent–catalyst was developed using the wet-mixing/sonication technique and applied for hydrogen production by sorption-enhanced steam methane reforming (SESMR), an intensified process that integrates hydrogen production with CO₂ capture. The material was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and N₂ physisorption (BET). CO₂ sorption efficiency of the developed materials was evaluated during 25 CO₂ sorption/regeneration cycles. The prepared sorbent–catalysts were then applied in the SESMR during 10 reaction cycles. The results showed that the bifunctional sorbent–catalyst with 20.5 wt% NiO loading presented the most suitable activity. The H₂ yield of ∼91% at the end of the 10th SESMR cycle is considerably higher than equilibrium H₂ yield that could be obtained by traditional steam methane reforming.     

Reaction path of ethanol and acetic acid steam reforming over Ni–Zn–Al catalysts. Flow reactor studies

Ethanol steam reforming (ESR) experiments have been performed in dilute conditions over a NiZnAl catalyst. Experiments have been performed by varying catalyst surface area, reactants flow rate, contact time, reactants feed composition and temperature. Acetic acid steam reforming experiments have also been performed. The data suggest that adsorbed acetaldehyde and acetic acid play an important role as intermediates of ESR, while also acetone may have a role in the ESR reaction. The key step for high hydrogen yield during ESR is represented by the evolution of acetate species, either towards decomposition giving rise to methane + CO_x, or to steam reforming to CO₂ and H₂. At high temperature hydrogen production depends on approaching methane steam reforming and reverse water gas shift equilibria. Ethylene end dimethylether are parallel products found at low conversion. With excess water acetaldehyde is not found among the products, and hydrogen yields as high as 95% have been obtained at 853 K.     

锂基CO2吸附剂在吸附强化甲烷水蒸汽重整制氢中的应用研究进展

简介了吸附强化甲烷水蒸汽重整制氢的研究背景,重点介绍了新型锂基CO2吸附剂(包括Li2ZrO3、Li4SiO4、Na2ZrO3)在吸附强化甲烷水蒸汽重整制氢中的应用研究进展,对新型吸附剂在吸附强化制氢中的应用研究进行了展望。     

Sorption properties of lithium carbonate doped CaO and its performance in sorption enhanced methane reforming

In-house prepared lithium carbonate doped CaO was tested for its CO₂ sorption properties and suitability as a CO₂ sorbent for sorption-enhanced reforming of methane. The new material demonstrated CO₂ capacity at the temperatures above the equilibrium for CaO recarbonation reaction. However, the capacity was unstable and decreased during carbonation–regeneration cycles. After sufficiently large number of cycles Li dopant escaped from the sorbent and its sorption behavior resembled to that of CaO. The main route of escape is, probably, a crossover of liquid Li₂CO₃ onto crucible in TG experiments and onto catalyst in SER tests. Sorption enhanced methane reforming at 2 bar pressure, 750 °C and H₂O to CH₄ ratio of 4 using novel sorbent yielded as high as 99.8 vol% pure hydrogen during the first cycle. In subsequent cycles the hydrogen purity drastically decreased as a result of severe catalyst poisoning by Li.     

ALTERNATIVE GENERATION OF H2, CO AND H2, CO2 MIXTURES FROM STEAM-CARBON DIOXIDE REFORMING OF METHANE AND THE WATER GAS SHIFT WITH PERMEABLE (MEMBRANE) REACTORS

A new process is proposed which converts CO₂ and CH₄ containing gas streams to synthesis gas, a mixture of CO and H₂ via the catalytic reaction scheme of steam-carbon dioxide reforming of methane or the respective one of only carbon dioxide reforming of methane, in permeable (membrane) reactors. The membrane reformer (permreactor) can be made by reactive or inert materials such as metal alloys, microporous ceramics, glasses and composites which all are hydrogen permselective. The rejected CO reacts with steam and converted catalytically to CO₂ and H₂ via the water gas shift in a consecutive permreactor made by similar to the reformer materials and alternatively by high glass transition temperature polymers. Both permreactors can recover H₂ in permeate by using metal membranes, and H₂ rich mixtures by using ceramic, glass and composite type permselective membranes. H₂ and CO₂ can be recovered simultaneously in water gas shift step after steam condensation by using organic polymer membranes. Product yields are increased through permreactor equilibrium shift and reaction separation process integration.

CO and H₂ can be combined in first step to be used for chemical synthesis or as fuel in power generation cycles. Mixtures of CO₂ and H₂ in second step can be used for synthesis as well (e.g., alternative methanol synthesis) and as direct feed in molten carbonate fuel cells. Pure H₂ from the above processes can be used also for synthesis or as fuel in power systems and fuel cells. The overall process can be considered environmentally benign because it offers an in-situ abatement of the greenhouse CO₂ and CH₄ gases and related hydrocarbon-CO₂ feedstocks (e.g., coal, landfill, natural, flue gases), through chemical reactions, to the upgraded calorific value synthesis gas and H₂, H₂ mixture products.     

Production of hydrogen by unmixed steam reforming of methane

Unmixed steam reforming is an alternative method of catalytic steam reforming that uses separate air and fuel–steam feeds, producing a reformate high in H₂ content using a single reactor and a variety of fuels. It claims insensitivity to carbon formation and can operate autothermally. The high H₂ content is achieved by in situ N₂ separation from the air using an oxygen transfer material (OTM), and by CO₂ capture using a solid sorbent. The OTM and CO₂ sorbent are regenerated during the fuel–steam feed and the air feed, respectively, within the same reactor. This paper describes the steps taken to choose a suitable CO₂-sorbent material for this process when using methane fuel with the help of microreactor tests, and the study of the carbonation efficiency and regeneration ability of the materials tested. Elemental balances from bench scale experiments using the best OTM in the absence of the CO₂ sorbent allow identifying the sequence of the chemical reaction mechanism. The effect of reactor temperature between 600 and on the process outputs is investigated. Temperatures of 600 and under the fuel–steam feed were each found to offer a different set of desirable outputs. Two stages during the fuel–steam feed were characterised by a different set of global reactions, an initial stage where the OTM is reduced directly by methane, and indirectly by hydrogen produced by methane thermal decomposition, in the second stage, steam reforming takes over once sufficient OTM has been reduced. The implications of these stages on the process desirable outputs such as efficiency of reactants conversion, reformate gas quality, and transient effects are discussed.     

CO2 conversion through combined steam and CO2 reforming of methane reactions over Ni and Co catalysts

Ni‐Co bimetallic and Ni or Co monometallic catalysts prepared for CO₂ reforming of methane were tested with the stimulated biogas containing steam, CO₂, CH₄, H₂, and CO. A mix of the prepared CO₂ reforming catalyst and a commercial steam reforming catalyst was used in hopes of maximizing the CO₂ conversion. Both CO₂ reforming and steam reforming of CH₄ occurred over the prepared Ni‐Co bimetallic and Ni or Co monometallic catalysts when the feed contained steam. However, CO₂ reforming did not occur on the commercial steam reforming catalyst. There was a critical steam content limit above which the catalyst facilitated no more CO₂ conversion but net CO₂ production for steam reforming and water‐gas shift became the dominant reactions in the system. The Ni‐Co bimetallic catalyst can convert more than 70% of CO₂ in a biogas feed that contains ~33 mol% of CH₄, 21.5 mol% of CO₂, 12 mol% of H₂O, 3.5 mol% of H₂, and 30 mol% of N₂. The H₂/CO ratio of the produced syngas was in the range of 1.8‐2. X‐ray absorption spectroscopy of the spent catalysts revealed that the metallic sites of Ni‐Co bimetallic, Ni and Co monometallic catalysts after the steam reforming of methane reaction with equimolar feed (CH₄:H₂O:N₂ = 1:1:1) experienced severe oxidation, which led to the catalytic deactivation.     

H2 rich product gas by steam gasification of biomass with in situ CO2 absorption in a dual fluidized bed system of 8 MW fuel input

The steam gasification of solid biomass by means of the absorption enhanced reforming process (AER process) yields a high quality product gas with increased hydrogen content. The product gas can be used for a wide range of applications which covers the conventional combined heat and power production as well as the operation of fuel cells, the conversion into liquid fuels or the generation of synthetic natural gas and hydrogen. On the basis of a dual fluidized bed system, steam gasification of biomass is coupled with in situ CO₂ absorption to enhance the formation of hydrogen. The reactive bed material (limestone) used in the dual fluidized bed system realizes the continuous CO₂ removal by cyclic carbonation of CaO and calcination of CaCO₃. Biomass gasification with in situ CO₂ absorption has been substantially proven in pilot plant scale of 100 kW fuel input. The present paper outlines the basic principles of steam gasification combined with the AER process the investigations in reactive bed materials, and concentrates further on the first time application of the AER process on industrial scale. The first time application has been carried out within an experimental campaign at a combined heat and power plant of 8 MW fuel input. The results are outlined with regard to the process conditions and achieved product gas composition. Furthermore, the results are compared with standard steam gasification of biomass as well as with application of absorption enhanced reforming process at pilot plant scale.     

Mathematical Modeling and Optimization of Combined Steam and Dry Reforming of Methane Process in Catalytic Fluidized Bed Membrane Reactor

Mathematical modeling of the methane-combined reforming process (steam methane reforming–dry reforming methane) was performed in a fluidized bed membrane reactor. The model characterizes multiple phases and regions considering low-density phase, high-density phase, membrane, and free board regions that allow study of reactor performance. It is demonstrated that the combined effect of membrane and reaction coupling provides opportunities to overcome equilibrium limits and helps to achieve higher conversion. Additionally, the influence of key parameters on reactor performance including reactor temperature, reactor pressure, steam to methane feed ratio (S/C), and carbon dioxide to methane feed ratio (CO₂/C) were investigated in the multi-objective genetic algorithm to find the optimal operating conditions. Finally, the process of steam reforming was simulated in selected optimal conditions and the results are compared to those of the combined reforming process. Comparison reveals the superiority of the combined reforming process in terms of methane conversion, catalyst activity, and outlet H₂/CO ratio in the syngas product in being close to unity.     

Modelling and simulation of two-bed PSA process for separating H_2 from methane steam reforming

Methane steam reforming is the main hydrogen production method in the industry. The product of methane steam reforming contains H_2, CH_4, CO and CO_2 and is then purified by pressure swing adsorption(PSA) technology. In this study, a layered two-bed PSA process was designed theoretically to purify H_2 from methane steam reforming off gas. The effects of adsorption pressure, adsorption time and purgeto-feed ratio(P/F ratio) on process performance were investigated to design a PSA process with more than99.95% purity and 80% recovery. Since the feed composition of the PSA process changes with the upstream process, the effect of the feed composition on the process performance was discussed as well.The result showed that the increase of CH_4 concentration, which was the weakest adsorbate, would have a negative impact on product purity.     

Ru-catalyzed anode materials for direct hydrocarbon SOFCs

A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO₂, which avoided interference from steam and CO₂ in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm⁻² with dry methane, which was comparable to the peak power density of 769 mW cm⁻² with wet (2.9 vol.% H₂O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion process.     

Sorption-enhanced dimethyl ether synthesis—Multiscale reactor modeling

As an opportunity for the attenuation of atmospheric CO₂ emissions, conversion of carbon dioxide into valuable oxygenates as fuel additives or fuel surrogates was explored conceptually in terms of a potentially feasible dimethyl ether (DME) conversion process. Incentives for application of conventional CO₂–DME conversion process are insufficient due to low CO₂ conversion, and DME yield and selectivity. In-situ H₂O removal by adsorption (sorption-enhanced reaction process) can lead to the displacement of the water gas shift equilibrium and therefore, the enhancement of CO₂ conversion into methanol and the improvement of DME productivity. A two-scale, isothermal, unsteady-state model has been developed to evaluate the performance of a sorption-enhanced DME synthesis reactor. Modeling results show that under H₂O removal conditions, methanol and DME yields and DME selectivity are favoured and the methanol selectivity decreases. The increase of methanol and DME yields and DME selectivity becomes more important at higher CO₂ feed concentration because a relatively large amount of water is produced followed by a large quantity of water removed from the system. Also, the drop in the fraction of unconverted methanol becomes more important when CO₂ feed concentration is higher and the dehydration reaction is favoured. Therefore, application of the sorption-enhanced reaction concept allows the use of CO₂ as a constituent of the synthesis gas as the in-situ H₂O removal accelerates the reverse water gas shift reaction.     

Analyzing the energy intensity and greenhouse gas emission of Canadian oil sands crude upgrading through process modeling and simulation

This paper presents an evaluation of the energy intensity and related greenhouse gas/CO₂ emissions of integrated oil sands crude upgrading processes. Two major oil sands crude upgrading schemes currently used in Canadian oil sands operations were investigated: cokingbased and hydroconversion-based. The analysis, which was based on a robust process model of the entire process, was constructed in Aspen HYSYS and calibrated with representative data. Simulations were conducted for the two upgrading schemes in order to generate a detailed inventory of the required energy and utility inputs: process fuel, steam, hydrogen and power. It was concluded that while hydroconversion-based scheme yields considerably higher amount of synthetic crude oil (SCO) than the cokerbased scheme (94 wt-% vs. 76 wt-%), it consumes more energy and is therefore more CO₂-intensive (413.2 kg CO₂/m³ SCO vs. 216.4 kg CO₂/m³ SCO). This substantial difference results from the large amount of hydrogen consumed in the ebullated-bed hydroconverter in the hydroconversion-based scheme, as hydrogen production through conventional methane steam reforming is highly energy-intensive and therefore the major source of CO₂ emission. Further simulations indicated that optimization of hydroconverter operating variables had only a minor effect on the overall CO₂ emission due to the complex trade-off effect between energy inputs.